The present invention relates to an apparatus for measuring structures on a mask and for calculating the structures in a photoresist on a wafer resulting from the structures on the mask.
The present invention further relates to a method for measuring structures on a mask and for determining the structures in a photoresist on a wafer resulting from the structures on the mask.
A measuring device according to the prior art measures structures on a substrate, wherein the measuring device comprises incident-light and/or transmitted-light illumination means, an imaging means and a detector means for the imaged structures, and a measuring stage interferometrically controlled and translatable in a measured fashion in a plane vertical to the optical axis of the imaging means and relative to it for receiving the substrate. The measuring device is not suited, however, to determine how the structures on the substrate (mask) are ultimately imaged in the photoresist on the wafer by the stepper.
A measuring device of this type was described in detail, for example, in the text of the paper entitled “Pattern Placement Metrology for Mask Making”, presented by Dr. Carola Bläsing at the Semicon meeting, Education Program in Geneva, Switzerland on Mar. 31, 1998. The measuring device is set up in a climate chamber in order to achieve measurement accuracy in the nanometer range. The position coordinates of various structures or features on masks and wafers are measured. The measuring system is mounted on a vibration-damped granite block. The masks and wafers are placed on the measuring stage by an automatic handling system.
German Patent Application DE 19819492 discloses a measuring machine for structural widths or the positions of structures on the substrate. The measuring stage glides on air bearings on the surface of the granite block. Planar mirrors are mounted on two mutually vertical sides of the measuring stage. A laser interferometer system determines the position of the measuring stage. Different guiding of the measuring stage is also conceivable, which is suitable for clean room environments. The illumination and imaging of the structures to be measured is carried out by a high-resolution, apo-chromatically corrected microscope optics in the incident-light or transmitted-light modes in the spectral range of the near UV. A CCD camera serves as a detector. Measuring signals are obtained from the pixels of the detector array arranged within a measuring window. An intensity profile of the measured structure is derived by means of image processing, from which the edge position of the structure can be determined, for example.
The measured edge position is on the one hand dependent on the physical quality of the edge itself and on the other hand on the optical measuring method used, as well as on the quality of the imaging system. These relationships are described in the paper “Edge Measurement on Microstructures”, W. Mirandé, VDI Berichte Nr. 1102 (1993), pages 137 et seq. If the position of the structure is defined by the center line between the two edges, influences on the measured edge position have generally no effect on the measured position of the structure. The evaluation of the measuring results for a structure width measurement can lead, however, to different results in different measuring devices.
In semiconductor manufacture, the mask is illuminated in the stepper with transmitted light and imaged on the wafer. It is therefore of particular interest to be able to determine the precise opaque width of the structural element. Special measuring microscopes have been developed, wherein the mask is illuminated with transmitted light and only the width of the opaque structure image is measured. These measuring devices are not for determining the position coordinates of the structural elements. These considerations apply in the same manner if transparent structural elements instead of opaque structural elements are to be measured in the mask surface.
German Patent Application DE 10 2005 009 536 A1 discloses a method for mask inspection, which can be used in the context of the mask design of mask manufacture, so that relevant problematic points can be detected and corrected at an early stage. As a result, it should be possible to detect and eliminate defects already in the mask layout and mask design, so that the masks to be made have a lower total number of defects and costs are minimized.
For the analysis of mask defects with respect to printability, the AIMS<TM>(Aerial Imaging Measurement System) of Carl Zeiss SMS GmbH has been established in the market for ten years. Herein, a small area of the mask (defect position with surroundings) is illuminated and imaged with the same illumination and imaging conditions (wavelength, NA (numerical aperture), type of illumination, degree of coherence of the light (sigma)) as in the lithographic scanner. Unlike the process in the scanner, the aerial image generated of the mask is enlarged onto a CCD camera. The camera sees the same latent image as the photoresist on the wafer. This is how the aerial image can be analyzed without cumbersome test prints by wafer imaging devices, and it is possible to draw conclusions as to the printability of the mask. By recording a focus series, additional information for analyzing the lithographic process window is obtained (cf. DE 10332059 A1).
German Patent Application DE 10332059 A1 discloses a method of analyzing objects in micro-lithography, preferably masks, by means of an aerial image measurement system (AIMS), comprising at least two imaging stages, wherein the detected image is corrected with respect to the transmission behavior of the second or further imaging stages by means of a correction filter, and illuminating the object is carried out in the incident-light and/or transmitted-light modes, wherein the correction is carried out in such a way that the corrected output values correspond to the image of a photolithography stepper or scanner, wherein the correction is carried out by reverse convolution, and measured or calculated correction values are used for the correction.
European Patent Application EP 0 628 806 discloses an apparatus and a method for determining the characteristics of a photolithographic mask. In the AIMS mask inspection microscope, this involves, for example, the adjustment and observation of certain illumination settings. The light for illumination is taken from the UV range. The detector or the imaging unit is a UV CCD camera.
In International Patent Application WO 00/60415 A1, a method for correcting imaging errors is described, wherein, by changing an electronic mask design after illuminating this mask design through a mask printer, structures are imaged on the mask, which come as close as possible to the original mask design or setpoint mask. The process conditions to be observed are set out in the form of tables comprising, in particular, the parameters dependent on the process conditions in the form of correction values. For example, various correction values for compensating the device dependent aberration of the mask printer are included in the table in a location dependent manner. The approach uses physically based models of the corresponding imaging errors as a precondition for all correction values. While it is possible with the suggested method, unlike prior art methods, to correct effectively mask structures for manufacturing large-scale integrated circuits, computation overhead is considerable. A further drawback of this method is the great number of parameters to be taken into account in the form of correction values. Apart from diffraction and refraction effects, interaction effects and device-dependent aberration effects are also to be taken into account.
Hitherto unpublished German Patent Application DE 10 2007 028 260 discloses an apparatus for measuring the positions and structural widths of at least one structure on a surface of a substrate. The substrate is inserted in the measuring stage in such a way that a normal vector extending from the surface of the substrate carrying the structures is essentially parallel to the vector of gravitational force.
None of the apparatus and/or methods known from the state of the art can determine the position or CD (critical dimension) of structures on a mask in one and the same apparatus and also predict the result to be obtained if the mask were to be imaged in a photoresist on the wafer by means of a stepper.